Simple Theory Versus Complex Real Ecosystems

Biomanipulation of lake ecosystems is based on the 'food chain theory,' which, for inland waters, states that phytoplankton are eaten by Zooplankton, which are eaten by planktivorous fish, which, in turn, are eaten by piscivorous fish (Figure 1). If a lake is undergoing eutrophication, excess supply of nutrients enters the food chain and promotes rapid and high

Piscivorous fish

Planktivorous fish

Zooplankton

Phytoplankton

Figure 1 A simplified food chain, constituting the theoretical basis for biomanipulation. The food chain starts with an increased nutrient input ('eutrophication'), which makes the phytoplankton (algae) abundant. Zooplankton feed on algae and planktivorous fish on zooplankton. Finally, piscivorous fish feed on other fish.

Piscivorous fish

Planktivorous fish

Zooplankton

Phytoplankton

Figure 1 A simplified food chain, constituting the theoretical basis for biomanipulation. The food chain starts with an increased nutrient input ('eutrophication'), which makes the phytoplankton (algae) abundant. Zooplankton feed on algae and planktivorous fish on zooplankton. Finally, piscivorous fish feed on other fish.

phytoplankton growth so that they become very abundant. In theory, the increasing amount of phytoplankton will provide abundant food supply for the zooplankton community, which, in turn, will provide excess food for planktivorous fish, and which will finally cause an increase in the next level of the food chain, piscivorous fish (Figure 1). Thus, if nutrients are provided at the bottom of the food chain, we would, logically, get a bottom up increase in abundances at all trophic levels in the food chain. In aquatic systems this is, however, not as simple since top predators (piscivorous fish) are visual feeders, i.e., they need to see their prey in order to catch it. As the water becomes green and turbid, their hunting success decreases and eventually the piscivorous fish cannot regulate the trophic level below the plankti-vorous fish. These planktivores then have excess food and are not efficiently preyed upon by the piscivorous fish, and therefore they become very abundant. Hence, a common and complex problem in lakes undergoing eutrophication is that the water becomes green and the planktivorous fish, such as cyprinids, become abundant, whereas zooplankton and piscivorous fish become much less conspicuous (Figure 1). The logic behind this chain of events is appealing and easy to understand. Especially appealing is the prediction that if planktivorous fish are removed, or piscivorous fish added, zooplankton will be released from the predation pressure allowing them to increase their grazing pressure on the phytoplankton, thereby causing lake water to become clear.

The relative simplicity of the theory, and the clear predictions derived from it, have attracted considerable interest from researchers and engineers. The road from basic science to its application is, however, not always straight. Although predictions from theory may be correct, the response of natural ecosystems subject to biomanipulation has often been more complex than expected or desired, and thus not always successful. Hence, some biomanipulations have failed to reach their predicted goals as many unexpected problems have arisen. On the other hand, there have also been several unexpected gains. Obviously, therefore, the original basis for performing biomanipulations - the theory of food chain manipulation - may not suffice to explain the complex responses of natural ecosystems. In the following we attempt to unravel some of this complexity.

Resolving the Complexity

The application of food chain theory to lake biomanipulation has been surprisingly fast and many studies using this approach have been performed to restore eutrophicated lakes and to prevent algal bloom nuisance that often accompanies eutrophica-tion of lakes. A major lesson learnt from the first biomanipulations is that the predictions from the food chain theory are not the only processes involved and that the theory may be too simple for a complex reality. Removal of planktivorous fish from a lake or addition of piscivorous fish to a lake, or both, clearly have effects on lower trophic levels, but not necessarily to the extent as predicted by the simple and appealing food chain theory. The importance of processes other than pure food chain dynamics has been demonstrated mainly in shallow lakes where the theoretical developments have progressed in closer cooperation with lake managers than in deep lakes. In many of the biomanipulation experiments performed, strong reductions in phytoplankton biomass have been observed due to increased grazing pressure from zooplankton. However, as zooplankton abundance often decreases, algal blooms often return, i.e., the effects are sometimes transitory and difficult to sustain over longer periods.

The risk of increased recruitment of young fish A

major cause for these, sometimes short-lived, positive effects of biomanipulations is the increased recruitment of young fish. When the abundance of cyprinid fish is reduced through biomanipulation, competition for zooplankton food decreases and the recruitment of strong year classes of young fish is enhanced. Even though they are very small, the young fish grow fast, thereby mobilizing and excreting high amounts of nutrients. Hence, high recruitment of young fish and their predation on zooplankton may strongly counteract or offset the effect of the biomanipulation (Figure 2). This 'baby boom' of fish is commonly observed 1-4 years after a biomanipulation and the young fish strongly reduce the abundance of large, efficient herbivorous zooplankton. Hence, the very high reproductive potential of fish is, indeed, a problem that has to be considered when planning a biomanipulation. Possible solutions are additional fish reductions, prevention of recruitment by egg destruction, or addition of piscivorous fish to the lake.

The feeding of benthic fishes As already stated, the theory behind biomanipulation rests mainly on pelagic processes, such as fish predation on zooplankton and zooplankton grazing on phytoplankton, whereas the importance of bottom feeding fish species is not always considered. Specialized benthic feeding fish, such as bream (Abramis brama) and gizzard shad (Dorosoma cepedianum), are 'vacuum cleaning' the sediment surface in their search for prey, such as chironomid larvae. This feeding behavior causes resuspension of sediment particles, which in shallow lakes reduces the light penetration in the entire water column decreasing the foraging efficiency of piscivorous fish even more. Although most planktivorous fish mainly feed on zooplankton, they are forced to switch to feeding on benthic animals when zooplankton become scarce. Thus, such a shift from planktivorous to benthic feeding causes bioturbation and resuspension of sedimented material and also results in increased leakage of nutrients from the sediment and their diffusion to the overlying water layer, providing phytoplankton with additional nutrient resources. Hence, there is a strong argument for reducing the abundance not only of planktivor-ous, pelagic fish, but also of benthic feeding fish.

Expansion of submerged macrophytes A general attribute of most lakes is that large amounts of phy-toplankton and submerged macrophytes only rarely occur simultaneously. Moreover, after a more successful biomanipulation, the cover of submerged macrophytes generally increases at the expense of phytoplankton (Figure 2). There are several factors that explain the macrophyte expansion: the main ones are (1) increased light availability due to zooplankton grazing on the phytoplankton and (2) reduction of direct, physical disturbance of the sediment and macrophytes by the benthic feeding fish.

Figure 2 During a biomanipulation, processes other than the feeding links within the food chain are active (the removal of fish is illustrated by crossed-over fish). For example, reduced competition between planktivorous fish is a risk leading to strong survival of young fish which may shortcut the expected decrease in predation pressure on zooplankton. Positive processes and opportunities may be reduced bioturbation by benthic feeding fish, which leads to improved conditions for submersed macrophytes and reduced internal loading of nutrients; processes leading to increased water clarity. Moreover, reduced predation by fish also results in higher amounts of benthic invertebrates, which, in turn, improves the conditions for waterfowl.

Figure 2 During a biomanipulation, processes other than the feeding links within the food chain are active (the removal of fish is illustrated by crossed-over fish). For example, reduced competition between planktivorous fish is a risk leading to strong survival of young fish which may shortcut the expected decrease in predation pressure on zooplankton. Positive processes and opportunities may be reduced bioturbation by benthic feeding fish, which leads to improved conditions for submersed macrophytes and reduced internal loading of nutrients; processes leading to increased water clarity. Moreover, reduced predation by fish also results in higher amounts of benthic invertebrates, which, in turn, improves the conditions for waterfowl.

Moreover, submerged macrophytes and the algae attached to them are able to absorb large quantities of nutrients during the summer season, thereby out-competing the phytoplankton. Macrophytes also stabilize the sediment surface, reducing resuspension of sediment particles, thereby improving the underwater light climate. Macrophytes may also function as a refuge for zooplankton from fish predation, and they are reported to favor predatory fish such as pike (Esox esox), more than cyprinids, such as roach (Rutilus rutilus) and bream (A. brama).

Reduced nutrient concentrations At low nutrient concentrations, lake water is generally clear, macro-phytes are abundant and piscivores such as pike are common in the fish community. As nutrient concentrations increase, phytoplankton and planktivorous fish become abundant, macrophytes become rare or even disappear if the nutrient concentrations further increase. Such lakes are generally dominated by cyano-bacteria or green algae and planktivorous fish. Several studies show that total phosphorus concentration should be less than 100 mg phosphorus to obtain a long-term, i.e., sustainable effect of biomanipulation in shallow lakes. However, a biomanipulation may be performed at higher phosphorus concentrations and still be successful as the manipulation often leads to a reduction in total phosphorus, at least temporarily. Such reductions have been reported in the literature and are most probably a combined result of reduced internal loading of phosphorus from the sediments, lower abundances of bottom feeding fish and phosphorus absorption by macrophytes. In addition, improved light climate at the sediment surface stimulates peri-phytic algal growth (algae growing at surfaces such as macrophyte leaves), and thereby oxygen production through photosynthesis, chemical sorption, and biological uptake of phosphorus at the sediment surface.

Benthic invertebrates and waterfowl Biomanipulation generally also leads to a strong increase in the amount of benthic invertebrates at the sediment surface (Figure 3), mainly due to reduced activities of benthic fish. Moreover, an expansion of submersed macrophytes (explained earlier) stimulates the growth

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Figure 3 The figure illustrates the positive effects on both macroinvertebrates and waterfowl following biomanipulation. Number of benthic invertebrates (no. per square meter) and the tufted duck (Aythia fuligula) in the western basin of Lake Ringsjon, Sweden, before eutrophication (until about 1970), during the severe eutrophication period (from about 1971 onwards), to the mid-1990s. The arrows indicate the period of biomanipulation efforts.

of benthic invertebrates even further. As benthic invertebrates and submersed macrophytes become more abundant following a biomanipulation, new food resources become available for waterfowl, such as coot (Fulica atra), swans (Cygnus spp.), and diving ducks (Figure 3). This causes such lakes to shift from a state characterized by high abundances of plankti-vorous fish and algal blooms into lakes with well-structured meadows of submerged macrophytes, clear water, and numerous waterfowl. Hence, the recreational value may increase considerably.

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